In a mobile communication system, effective utilization of radio spectrum resources requires a speech signal to be compressed to a low bit rate before being transmitted. On the other hand, it is desired to improve the telephone speech quality and achieve a highly realistic communication service. In order to achieve such a communication service, it is desirable to perform high-quality encoding of a speech signal, a music signal, and the like with a broader frequency band.
A technique that hierarchically integrates a plurality of encoding techniques is regarded as promising for achieving these conflicting demands. This technique hierarchically combines a first layer that encodes an input signal up to a wide band (0 kHz to 7 kHz) and an extended band layer that encodes a super-wide band (7 kHz to 16 kHz) by using the input signal and the decoded signal of the first layer. Hereinafter, the frequency band encoded by the first layer (0 kHz to 7 kHz) is called the wide band part, and the frequency band encoded by the extended band layer (7 kHz to 16 kHz) is called the extended band part. FIG. 1 shows the wide band part and the extended band part in the input signal spectrum.
Because the technique performing such hierarchical encoding has scalability of a bit stream outputted from an encoding apparatus, that is, has the characteristic of being able to obtain a decoded signal from the information of even a part of the bit stream, it is generally referred to as scalable encoding (layer encoding).
Because of the nature of scalable encoding that enables flexible communication between networks having different bit rates, scalable encoding can be said to be suited for a future network environment, in which diverse networks are integrated by IP protocols.
An example of the implementation of scalable encoding using a technique that has been standardized by ITU-T (International Telecommunications Union Telecommunication Standardization Sector) is disclosed in Non-Patent Literature 1. This technique encodes a wide band signal in the first layer and performs encoding in an extended band layer by generating a signal of the extended band layer from a signal of the wide band layer. Using this type of scalable configuration can achieve high sound quality of decoded speech signal and a music signal which has a wider frequency band than a speech signal has.
In the case of an encoding system that encodes a signal up to a super-wide band to achieve high sound quality, because the frequency band of the signal is wide and the amount of encoding information is large, the bit rate becomes high. In wireless communication, because the bit rate allocated for speech communication is limited, there is a need to make the bit rate for speech communication as low as possible. In general, since the radio spectrum resources in wireless communication are limited, it is necessary to restrict the communication channel capacity on individual links. Therefore, the total bit rate for a speech codec is to be restricted to approximately 16 kbps.